Re: [Qemu-devel] migration: new sections and backward compatibility.

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/06/2011 12:28 PM, Avi Kivity wrote:
> On 07/06/2011 07:04 PM, Gerd Hoffmann wrote:
>> Hi folks,
>>
>> We'll need to figure a sane way to handle migration to older versions
>> with new sections, i.e. devices which used to not save state before do
>> now.
>>
>> We already have one case in tree: usb. qemu 0.14 saves state for
>> usb-hid devices and the usb-hub, whereas qemu 0.13 and older don't.
>> You can't migrate a vm with a usb-tablet from 0.14 to 0.13 because of
>> that even if you use -M pc-0.13.
>>
>> More cases are lurking. AHCI doesn't support migration today but
>> probably will some day. Markus mentioned that scsi-disk will face that
>> issue too. And that list probably isn't complete.
>>
>> Subsections don't help here as there is no toplevel section in the
>> first place.
>>
>> Ideas anyone? Maybe allow test functions like we have for subsections
>> for toplevel sections too, so we have a way to skip the section
>> altogether on savevm?
>>
>> We probably also want a way to fail the migration in case the target
>> machine doesn't support migration for $device, especially for $device
>> == ahci to avoid data loss. For the usb-tablet it isn't that
>> problematic, in the best case the guest just resets the device and
>> goes on, in the worst case the mouse is dead.
>>
>
> How did AHCI get in without migration? It's relatively new, is it not?

We don't have a hard policy about not merging devices that don't support 
migration.

Since migration must be supported forever, I'd rather see a device get 
some solid testing before it starts doing live migration.  That said, we 
should probably do this consciously by explicitly marking the device 
non-migrateable.

Regards,

Anthony Liguori

>